Method of adhering silicone rubber to metal surfaces using glycine or dimethylformamide

ABSTRACT

The adhesion to metal surfaces of room temperature vulcanizable silicone rubber curable through condensation of silicon-bonded hydroxyls with silicon-bonded alkoxy radicals is improved by contacting the metal surface with dimethyl-formamide or an aqueous solution of glycine, removing the glycine or dimethylformamide, drying, thereafter applying the room temperature vulcanizable silicone rubber to the treated surface and allowing it to cure.

United States Patent [1 1 Greenlee Oct. 30, 1973 [75] Inventor: Thomas W. Greenlee, Midland,

Mich.

[73] Assignee: Dow Corning Corporation, Midland,

Mich.

22 Filed: June 18, 1971 21 Appl. No.: 154,646

[52] U.S. Cl 117/49, 117/132 BS, 117/135.1 [51] Int. Cl. B4411 l/40, 844:! 1/36 [58] Field of Search 117/49, 132 BS, 135.1;

[56] References Cited UNITED STATES PATENTS 3,591,432 7/1971 Vazirini 117/49 X 3,607,769 9/1971 Page 134/24 X 3,065,194 11/1962 Nitzche et al 260/37 2,999,077 9/1961 Nitzche et al..... 260/18 3,127,363 3/1964 Nitzche et a1 260/18 2,607,739 8/1952 DeLong 134/41 X 2,302,939 11/1942 DeLong 134/41 X 3,444,007 5/1969 Maurer et a1. 148/6.l5 2,468,012 4/1949 Isbell 252/546 UX 2,063,987 12/1936 Dreyfus 252/546 X 3,179,609 4/1965 Morison 252/D1G. 8

OTHER PUBLICATIONS Burns et al., Protective Coatings for Metals, pps. 22-25, 425 and 427 (1955).

Primary Examiner-William D. Martin Assistant Examiner-I-Iarry J. Gwinnell Attorney-Robert F. Fleming, Jr. et al.

[5 7 ABSTRACT The adhesion to metal surfaces of room temperature vulcanizable silicone rubber curable through condensation of silicon-bonded hydroxyls with silicon-bonded alkoxy radicals is improved by contacting the metal surface with dimethyl-formamide or an aqueous solu tion of glycine, removing the glycine or dimethylformamide, drying, thereafter applying the room temperature vulcanizable silicone rubber to the treated surface and allowing it to cure.

4 Claims, No Drawings METHOD OF ADI-IERING SILICONE RUBBER T METAL SURFACES USING GLYCINE OR DIMETI-IYLFORMAMIDE This invention relates to a method of improving the adhesion of a room temperature vulcanizable silicone rubber to a metal surface.

The adhesion of silicone rubber to metal surfaces and particularly certain silicone rubbers to metal surfaces is a problem which has received considerable attention in the prior art. In 1944, a method was described in US. Pat. No. 2,354,011 which applied a layer of insoluble heavy metal salts of aliphatic fatty acids to metal surfaces to which an organic rubber was to be bonded. Metal salts of fatty acids usually act as release agents for silicone rubber and thus a number of other methods for adhering silicone rubber to metals have been developed. The methods known in the art to adhere silicone rubber to metal surfaces involve the application of an adhesive composition, a primer and the like, such that the silicone rubber bonds to another surface instead of a metal surface.

An object of the present invention is to bond silicone rubber, particularly room temperature vulcanizable, to metal surfaces without using an intervening layer of primer or adhesive. This object and others will become more apparent from the following detailed description of the present invention.

This invention relates to a method of improving the adhesion of room temperature vulcanizable silicone rubber to a metal surface comprising contacting a metal surface with dimethylformamide or an aqueous solution of glycine, removing the glycine or dimethylformamide from the metal surface, drying the metal surface wherein a treated metal surface is obtained, applying to the treated metal surface a room temperature vulcanizable silicone rubber consisting essentially of a hydroxyl endblocked polydiorganosiloxane having a viscosity of from 1000 to 100,000 cs. at 25C. and having organic radicals selected from the group consisting of methyl, ethyl, phenyl and 3,3,3-trifluoropropyl wherein at least 50 percent of the organic radicals are methyl radicals, an alkoxysilicon compound crosslinking agent and a curing catalyst, allowing the room temperature vulcanizable silicone rubber to cure whereby the adhesion of the room temperature vulcanizable silicone rubber to the treated metal surface is increased compared to the adhesion of the same room temperature vulcanizable silicone rubber applied to an untreated metal surface.

The adhesion of room temperature vulcanizable silicone rubber to a metal surface is improved by first contacting the metal surface with dimethylformamide or an aqueous solution of glycine. The metal surface can be metals such as iron, steel, stainless steel, aluminum, titanium, copper, tin and alloys thereof. Any method of contacting the metal surface with glycine or dimethylformamide can be used so long as an intimate contact is accomplished. The metal surface is preferably immersed in dimethylformamide, an aqueous solution of dimethylformamide or an aqueous solution of glycine. Other methods, which are suitable, include methods such as allowing dimethylformamide or the aqueous solutions of dimethylformamide or glycine to pass over the metal surface, wetting a sponge or cloth with the treating materials and contacting the surfacewith the wet applicator and the like. The time of contact can vary broadly from about one minute up to 64 hours or that temperatures which decompose or volatilize the treating material are not recommended. The temperature can conveniently be ambient temperatures.

The concentration of the glycine or the dimethylformamide in the aqueous solutions can vary broadly such as from one weight percent up to the saturation concentration.

After the metal surface has been contacted with the dimethylformamide or the aqueous solutions for a suitable time, the metal surface is rinsed with water to remove the glycine or dimethylformamide or in the case of the dimethylformamide the removal can be by evaporation. The method of rinsing can be immersion of the metal surface in water, water can be run over the metal surface and the like. After the metal surface is rinsed with water, the surface is dried, in the case of the dimethylformamide the removal step and the drying step can be the same. The drying can be done by merely allowing the metal surface to be exposed to ambient air. Other drying atmospheres can also be used such as nitrogen, a vacuum or the like. The atmosphere can be heated, but temperatures above C. or C. should not be used. The drying can be accomplished faster by rinsing away the water or dimethylformamide or glycine with a volatile solvent such as acetone or methanol. Exposure of the metal surface to a drying atmosphere for periods of up to 10 days does not effect the ultimate adhesion of the room temperature vulcanizable silicone rubber to the metal surface.

To the treated metal surface, a room temperature vulcanizable silicone rubber is applied. The particular room temperature vulcanizable silicone rubber of this invention is one which cures through the condensation of alkoxys bonded to silicon and hydroxyls bonded to silicon in the presence ofa curing catalyst. These room temperature vulcanizable silicone rubbers are well known in the art and are available commercially. These room temperature vulcanizable silicone rubbersv are known to possess a very low affinity for bonding to metal surfaces and therefore it is important to improve the adhesion to metal surfaces to broaden their utility. The elimination of adhesive compositions and primers can also be accomplished by treating the metal surface in accordance with this invention. Although the improvement in adhesive bond strength is not always an ultimate value, completely cohesive, sufficient enhancement of the adhesive bond strength is realized to make these room temperature vulcanizable silicone rubbers useful where adhesion to metal surfaces is needed.

The room temperature vulcanizable silicone rubbers consist essentially of a hydroxyl endblocked polydiorganosiloxane having a viscosity of from 1000 to l00,000 cs. at 25C., preferably from 1000 to 50,000 cs. at 25C. The organic radicals of the polydiorganosiloxane can be methyl, ethyl, phenyl and 3,3,3- trifluoropropyl where at least 50 percent of the organic radicals are methyl radicals. The hydroxyl endblocked polydiorganosiloxanes can be illustrated by polydimethylsiloxane, polymethyl-3,3,B-trifluoropropylsiloxane, polyethylmethylsiloxane, polydiethylsiloxane and polydiorganosiloxanes which are copolymers of two or more units of dimethylsiloxane units, diphenylsiloxane units, methylphenylsiloxane units, diethylsiloxane units and 3,3,3-trifluoropropylmethylsiloxane units. Mixtures and blends of polydiorganosiloxanes can also be used.

The crosslinking agent can be an alkoxysilicon compound which can be illustrated by orthosilicates such as ethylorthosilicate, n-propylorthosilicate, amylorthosilicate, pentenylorthosilicate, diethyldipropylorthosilicate and hexylorthosilicate; alkylpolysilicates such as methylpolysilicate, ethylpolysilicate, isopropylpolysilicate, n-propylpolysilicate, secondary amylpolysilicate and n-butylpolysilicate; monoorganotrialkoxysilanes such as methyltrimethoxysilane, ethyltriethoxysilane,

propyltrimethoxysilane, methyltriisopropoxysilane phenyltrimethoxysilane, vinyltrimethoxysilane; and other alkoxysilicon compounds such as Si(OCH CH OCI-l The curing catalyst can be illustrated by metal salts of carboxylic acids such as lead naphthenate, cobalt naphthenate, zinc naphthenate, sodium naphthenate, iron-2-ethylhexoate, lead 2- ethylhexoate, chromium octoate, dibutyltin dibenzoate, dibutyltin adipate, lead sebacate, dibutyltin dilactate, dibutyltindiacetate, dibutyltindilaurate, lead octoate, tin ricinoleate, cobalt hexoate, aluminum acetyl-acetonate, zirconium acetoacetate, dioctyltindimaleinate, stannous octoate and the like.

The above room temperature vulcanizable silicone rubbers are well known in the art and are available commercially. The room temperature vulcanizable silicone rubbers which comprise a hydroxyl endblocked polydiorganosiloxane, an alkoxysilicon compound and a curing catalyst are further defined in the following patents with respect to ingredients, amounts and methods of preparation. The following patents are hereby incorporated by reference and include U.S. Pat. No. 2,843,555 by Berridge, U.S. Pat. No. 2,902,467 by Chipman, U.S. Pat. No. 2,927,907 by Polmanteer, U.S. Pat. No. 3,065,194 by Nitzsche et al., U.S. Pat. No. 3,070,559 by Nitzsche et al., U.S. Pat. No. 3,070,566 by Nitzsche et al., U.S. Pat. No. 3,127,363 by Nitzsche et al. and U.S. Pat. No. 3,305,502 by Lampe. The room temperature vulcanizable silicone rubbers can also contain any of the fillers and additives which are well known and described in the prior art.

After the room temperature vulcanizable silicone rubber is applied to the treated metal surface, it is allowed to cure. Since these silicone rubbers vulcanize when mixed at room temperature additional curing procedures are not required. However, one can increase the temperature to shorten the curing time, but this is not necessary since these silicone rubbers usually set up within a few hours. After the room temperature vulcanizable silicone rubber cures, the adhesion to the metal surface is improved compared to the same silicone rubber in combination with an untreated metal surface.

The following examples are illustrative only and should not be construed as limiting the present invention which is properly delineated in the claims.

EXAMPLE 1 Aluminum panels and foil were initially cleaned by immersing and rubbing with paper wipers first under trichloroethylene and then under methyl isobutyl ketone. The cleaned aluminum substrates were allowed to dry and then immersed in the treating agent for the times specified and then followed by the removal of the treating agent as indicated in the table. After the aluminum substrates dried, the room temperature vulcanizable silicone rubber was applied with a spatula to the treated aluminum foil strips measuring 10 inches by 1 inch by 0.004 inch. These foil strips were applied to a treated aluminum panel (6 inches by 4 inches by 1/16 inch) having a 4 inch width with the rubber side of the foil strip to the aluminum panel and then rolled with a 10 pound hand roller to provide a thickness of 5 to 8 mils for the room temperature vulcanizable silicone rubber. The test assembly was then allowed to cure at ambient conditions one week before adhesion tests were made. Adhesion tests were also done on identical samples after curing for one month. The adhesion was determined by using conventional test equipment for the 180. peel test at 2 inches per minute jaw separation rate. The adhesion results were as shown in the table.

The room temperature vulcanizable silicone rubber was prepared by thoroughly mixing the following ingredients: parts by weight of a hydroxyl endblocked polydimethyl-siloxane fluid having a viscosity of about 12,500 cs. at 25C., 30 parts by weight ofa trimethylsiloxy treated fume silica tiller, 4 parts by weight of npropylorthosilicate and 0.3 part by weight of dibutyltindiacetate.

TABLE Adhesion, Peel Pounds Per Linear Inch Run No. Treatment After I After 1 week month 1. None (Control)* 2-5 2-6 2. Water for 16 hours* 6 7 3. 5 weight percent glycine in water l6 19 for 16 hours, rinsed with water, allowed to air dry 4. dimethylformamide for 64 hours, l6 I? allowed to air dry For comparative purposes That which is claimed is:

1. A method of improving the adhesion of room temperature vulcanizable silicone rubber to a metal surface consisting essentially of contacting a cleaned metal surface with dimethylformamide or an aqueous solution of glycine, removing the glycine or dimethylformamide from the metal surface, drying the metal surface wherein a treated metal surface is obtained, applying to the treated metal surface a room temperature vulcanizable silicone rubber consisting essentially of a hydroxyl endblocked polydiorganosiloxane having a viscosity of from 1000 to l00,000 cs. at 25C. and having organic radicals selected'from the group consisting of methyl, ethyl, phenyl and 3,3,3-trifluoropropyl wherein at least 50 percent of the organic radicals are methyl radicals, an alkoxysilicon compound cros slinking agent and a curing catalyst, allowing the room temperature vulcanizable silicone rubber to cure whereby the adhesion of the room temperature vulcanizable silicone rubber to the treated metal surface is increased compared to the adhesion of the same room temperature vulcanizable silicone rubber applied to an untreated metal surface.

2. The method in accordance with claim 1 wherein the metal surface is contacted by an aqueous solution of glycine.

3. The method in accordance with claim 2 wherein the glycine is removed by rinsing with water.

4. The method in accordance with claim 1 wherein the metal surface is contacted by dimethylformamide. 

2. The method in accordance with claim 1 wherein the metal surface is contacted by an aqueous solution of glycine.
 3. The method in accordance with claim 2 wherein the glycine is removed by rinsing with water.
 4. The method in accordance with claim 1 wherein the metal surface is contacted by dimethylformamide. 